MTHFR and juvenile idiopathic arthritis: These studies demonstrate that genetic variants of enzymes and transporters in the methotrexate pathway, such as methylenetetrahydrofolate reductase (MTHFR), aminoimidazole carboxamide adenosine ribonucleotide transformylase (ATIC) and ATP-binding cassette transporters (ABCs) are associated with methotrexate response in JIA: this topic has been reviewed recently (Schmeling et al., 2014).